Recently, solar cells are receiving public attention as a clean energy source of the next generation. In a solar cell module, a component such as a sheet for sealing the backside of solar cell which seals the backside of solar cell module is used and a substrate film is used for the component as such. Since a solar cell used in outside is used for a long period of time, the component therefor is to be also durable under natural environment. As to the component such as a substrate film for sealing the backside of solar cell, there is used fluorine film, polyethylene film or polyester film (cf. Patent Documents 1 and 2). As to a sheet for sealing the backside of solar cell, there is proposed a white sheet for sealing the backside of solar cell in order to enhance the photoelectric conversion efficiency of the solar cell device (cf. Patent Documents 3 to 7).
Further, for improving the properties of the solar cell system as a whole, there has been a strong demand for setting the system voltage high. Therefore, there has been an increasing demand for the electric insulation property to the sheet for sealing the backside of solar cell and there have been proposals for a sheet for sealing the backside of solar cell compatible with the system voltage of 1,000 V (cf. Patent Documents 8 and 9).
Furthermore, as mentioned in the above Patent Documents, a polyester film of high whiteness degree has been used for reflecting the sunlight and for enhancing the generation efficiency by the solar cell device. In order to prepare a polyester film of high whiteness degree, it is necessary to add a lot of particles to the polyester substrate. At that time, many thermal hystereses are added due to the fact that a raw material where two or more materials are preliminarily mixed or that melting time is made long even in the usual extruding step so as to make the dispersibility and mixing state thereof good. Therefore, the polyester resin is hydrolyzed and is apt to be deteriorated whereby there has been a problem of poor durability when it is used under high temperature and humidity. Accordingly, it has been common up to now that, as mentioned in the Patent Document 7, a white layer and a layer having durability are prepared as separate functional layers followed by laminating them and it has been difficult to satisfy both high reflectivity and durability by a single layer.
Still further, in recent years, the mode for use of solar cell is developing from just placing on a roof to a large-scale photovoltanic generator in desert regions, etc. and a solar cell module has been becoming to large size and high output of power. At that time, many solar cell devices are aligned in a sheet of solar cell module. When unbalanced generation characteristic happens for each of the solar cell devices, load is focused to a solar cell device having low generation characteristic whereby the temperature of such a solar cell device part becomes particularly high. In a solar cell module where such a hot spot is resulted, the material around the hot spot is exposed to high temperature and, as a result, that causes deterioration of the resin such as sealing resin or backside protecting membrane or inferiority such as detachment.